To assess whether serum free cortisol (SFF) responses to cosyntropin simulation test (CST) may more accurately assess adrenal function than total cortisol (TF), we recruited healthy volunteers (HV; n = 27), patients with primary and secondary adrenal insufficiency (n = 19 and n = 24, respectively), and subjects with Child-Pugh class A cirrhosis (CH; n = 15). Each received 250 g cosyntropin with measurement of ACTH and corticosteroid binding globulin (CBG) at time 0 and TF and SFF at 0, 30, and 60 minutes. Salivary cortisol was measured at all time points in CH subjects. Peak SFF and TF were significantly higher in HVs vs both AI groups (P < .05). Peak SFF and TF (6.8 g/dL vs 2.2 g/dL; 188 nmol/L vs 62 nmol/L; P < .01) were significantly higher in the secondary adrenal insufficiency vs primary adrenal insufficiency patients. The optimal peak SFF criterion to identify adrenal insufficiency patients vs HV was 0.9 g/dL (25 nmol/L) (sensitivity of 95%, specificity of 100%). Mean CBG and albumin levels were similar among all four groups. CH patients had a higher peak SFF than HV (2.4 vs 2.0 g/dL; P = .02. In the CH patients, peak salivary cortisol levels correlated well with peak SFF (rs = 0.84, P = .005). CBG levels were similar among the groups. These results provide normative data for SFF values in HV and AI during the CST. Normal CBG levels in mild cirrhosis did not affect the interpretation of the CST.